Calvinists say that if we do not believe what they believe, we do not believe that God is sovereign. In particular, to affirm free will means, for them, to deny the sovereignty of God. What does it mean that God is sovereign?
Calvinists love to change the meaning of words, and their favorite is indeed sovereignty. To be sovereign means to be the absolute legitimate authority. The dictionary I have in hand defines the term in this way: “To hold a power that is not subject to any higher authority”. Such a definition does not imply deterministic control on the part of the sovereign, let alone meticulous control of personal freedoms.
The example I find useful is the following: a king is sovereign on his own land, therefore he legislates, etc. Let us also say that his word is law. However, he has no meticulous control over his subjects; they can decide at any time to violate the law. When they do, they do not undermine the sovereignty of the king. On the contrary, precisely because the king is sovereign, he can now apply his law and punish the guilty subjects.
Biblically, we can look at the book of Deuteronomy, where God, as the legitimate ruler of Israel, promulgates the laws for His nation. However, it is also clear that God gives them the freedom to respect them or not, with consequences for both choices (e.g. Deuteronomy 28), exactly as described in the example above.
In short: God’s sovereignty does not require meticulous control over people, so there is no need to compromise with deterministic theism, that is, the false fatalistic view of Christianity that the West inherits from Augustine’s mistakes, which we find intact in the fathers of the Reformation.